Video teleconferencing systems which utilize the conventional television grey scale are known. For example, Bell Telephone has established video teleconferencing facilities between Los Angeles and San Francisco and also along the East Coast. Similarly, Satellite Business Systems has announced plans for a worldwide, satellite-linked system, and Xerox has announced the XTEN project, a satellite-linked system digital communication network across the United States. In each case, these systems call for image transmission with normal grey scale reduction, and in some instances full color. However, such systems require large bandwidth, and are thus expensive. This and other difficulties have served as deterrents to substantial use of video teleconferencing systems.
The standard method of digital image transmission, as is typically used in known videoteleconferencing systems, is by pulse code modulation where, for monochrome systems, each image frame is sampled over a grid of H horizontal and V vertical picture elements, or pixels. Each pixel is linearly quantized into a predetermined number of levels (B), to produce a uniform codeword of b (equal to log.sub.2 B) bits, which serves as a grey scale resolution factor. The spatial resolution of the image is provided by HV, while the temporal resolution of the image is provided by the number of frames (F) transmitted per second. These factors combine to require a data source rate (R) of EQU R=HVFb.
For an exemplary system, values of H, V, F and b might be, respectively, 100 pixels per frame row, 150 pixels per frame column, 15 frames/sec and 6 bits per pixel, which leads to a source rate R of 1.35.times.10.sup.6 bits/second. Such data rates impose unacceptable bandwidth requirements, making the use of image coding techniques necessary.
Various coding or data compression techniques are known in the art for reducing the required data rate. Differential pulse code modulation provides 2:1 compression compared to PCM and was selected by Bell Telephone Laboratories for its short haul video telephone transmission system; frame replenishment coding provides 6:1 compression and was proposed by Bell for long-haul transmission. Intraframe transform using either Hadamard or cosine transform coding systems also provides 6:1 data compression. Interframe transform using similar techniques can provide 24:1 data compression. Both intraframe and interframe transform techniques have been implemented for remotely piloted vehicle television transmission.
However, none of these state of the art techniques of data compression provides data rates--i.e., bandwidth--which are low enough to be acceptable for use in a video teleconferencing system in which data is transmitted over conditioned telephone lines, where the maximum data rate is 9600 baud. For example, the 24:1 data compression factor (compared to PCM) available with interframe transform still requires a data rate of 56,250 bits/second. Moreover, the obvious choice of reducing the factors of either temporal or spatial resolution does not yield acceptable results, for two reasons. First, such reduction is likely to result in unacceptable image quality; second, the data compression factors given previously will be reduced as spatial and temporal resolution is reduced because of the diminishing correlation of the remaining pixels. As a result there has been a need for data compression techniques and a video teleconferencing system which permit operation at 9600 baud.